This invention concerns a control for automatically locking out the lag load in a dual load system arranged in a lead-lag configuration in order to prevent energization of the lag load when certain predetermined conditions exist. In addition, the lockout system desirably insures that the lag load is automatically deenergized when such predetermined conditions occur. Such action is desirable to insure economic energy utilization in systems in which multiple loads are sequentially energized in order to meet certain load capacity requirements.
For example, in large commercial buildings, a dual chiller system is frequently used for meeting the cooling requirements of the building. It is common to set up dual chillers in a lead-lag configuration such that the lead chiller is started first when a cooling demand is present. If the building load conditions require additional cooling, the lag chiller is then started. In many building systems, the lag chiller is manually started by the building equipment operator. The operator's decision to start the chiller is generally based upon his experience concerning what conditions are appropriate for starting the lag chiller. In many instances, this human factor results in the lag chiller being started when the actual cooling demand does not require this additional capacity. Consequently, energy is wasted. Similarly, when both chillers are operating the operator will ultimately determine when the cooling requirements are such that the lag chiller can be deenergized. In some cases the human factor can also result in wasted energy.
It is thus desirable to provide an automatic lockout control system which will eliminate the inherent energy waste which results in manual energization and deenergization of the lag chiller.